The present invention relates to transport and acceleration systems for charged particles and more particularly to electrostatic systems utilizing transverse field focusing (TFF) for transport and acceleration.
It is anticipated that magnetically confined plasmas in fusion reactors can be heated to fusion temperatures, and maintained for sustained operation, by the injection of powerful beams of neutral hydrogen or deuterium atoms. The injected particles must be electrically neutral so that they will not be prematurely deflected by the strong magnetic fields in the reactor which confine the plasma. However, in order for the injected particles to have the required level of kinetic energy, they must initially have an electric charge to enable them to be accelerated to that level.
Considerable developmental work has been done on high-current negative ion sources and on neutralizing systems for converting high current negative ion beams to neutral atoms by electron stripping.
The present invention is particularly directed to apparatus that can be used to transport the high-current negative-ion beam from the negative-ion source to the neutralizer, and that can provide the desired acceleration to the beam.
The problems involved in providing suitable apparatus for this purpose are considerable.
For example, the very high ion current required is constituted by a tremendous number of ions of like charge which repel each other and cause the ion beam to diverge and expand. The transport and acceleration apparatus must be able to confine the beam against divergence and expansion on its travels through the system.
After the negative ion beam has been neutralized, it must follow a straight line trajectory into the fusion reactor through a penetration in the reactor shielding. Neutrons in the reactor can, of course, exit via the same shielding penetration. If the escaping neutrons are able to reach the negative ion source equipment, such equipment will become radioactive by neutron activation, and must be remotely maintained. In order to prevent this severe problem, the negative-ion beam transport apparatus must be capable of transporting the negative-ion beam through a maze path in the neutron shielding, before the beam is neutralized, so that there is no direct line of sight from the interior of the fusion reaction to the ion source and other apparatus which must be maintained free of neutron radiation.
For efficient operation, background gas molecules must be removed before acceleration of the beam to, and transport of the beam at, high-energy levels. In order to accomplish this, the transport apparatus must enable the beam to bend around corners to follow a maze path through the baffles of a differential pump while allowing gas molecules to exit freely from the transport apparatus for movement to the cryopump panels.
Very high acceleration voltages are required for fusion reactor applications, e.g., in the range of 400-800 kV. If such voltages are used in a one-stage accelerator, it is extremely difficult to prevent sparking between electrodes. Conventional multi-stage accelerators can reduce arcing break-down by reducing the voltage difference per stage, but with a significant sacrifice in beam current.